Power Consumption Of Vacuum Pumps

I am back on my blog after a wonderful holiday to Canada. I had a lot of fun staying in Vancouver and found BC to be a beautiful place.

Today's blog entry deals with quick but reliable estimate of the power consumption of vacuum pumps used in the chemical process industry. Some equations are presented below for the benefit of the readers:

Liquid Ring (NASH) pump:

BkW = 21.4*(SF)0.924

Rotary Piston Pump:

BkW = 13.5*(SF)1.088

where:

SF = 0.02-16 for "Liquid Ring (NASH) pump" and SF = 0.01-4 for "Rotary Piston Pump"

SF is defined as the "Size Factor" and can be calculated as follows:

SF = m / P; kg/ h/torr

where:

P = Operating pressure of system evacuated, torr (mmHga)
m = Air equivalent flow rate, kg/h

"m" can be calculated as follows:

m = G*sqroot((273.15+T)*28.96 / (293.15*MW))

where:

G = flow rate of the process gas being evacuated, kg/h
T = temperature of the process gas being evacuated, ⁰C
MW = molecular weight of the process gas being evacuated

The value of 293.15 in the equation indicates a reference temperature of 20 ⁰C for air equiva
lent flow which is recommended by the "Heat Exchange Institute" (HEI) for vacuum systems.

This concludes today's blog entry and I look forward to comments from the readers.

Regards,
Ankur.

udokas

Thanks Engr. Ankur

bertrand

Hi Ankur, Is power expressed in kW?

ankur2061

Hi Ankur, Is power expressed in kW?

bertrand,

Yes, it is. The term BkW means "Brake Kilowatt" similar to BHP.

Regards,

Ankur.

PrT-Omi

Hello Mr Ankur,

Thank you for putting up the article. I found it very useful.

I do have a query. In a thermal power plant, the condenser is generally kept under vacuum conditions. For vacuum generation, its either a steam ejector or a vacuum pump.

The question is, if I suggest to replace the steam ejector with a vacuum pump, how would i go about in estimating the capacity handled? (I know the desired vacuum condition, and if know the capacity handled, from the formula i can calculate the kW of the pump).

cmorris7777777

Dear Mr. Ankur,

I am looking for an article from Chemical Engineering on Pneumatic Conveying.  My reference is that article appeared in the Feb 17, 1975 magazine.

Any ideas on where I can get this?

btw I live in Ontario, Canada and this is a spectular country!

hollerg

Is the  the air equivalent flow rate m for these rotory compressors, the same as a DAE (dry air equivalent), whichspecified for vacuum jets, and if not how do they relate to one another?

ankur2061

Is the  the air equivalent flow rate m for these rotory compressors, the same as a DAE (dry air equivalent), whichspecified for vacuum jets, and if not how do they relate to one another?

hollerg,

The DAE calculations (English Units) that I have presented in an earlier blog entry are based on the HEI (Heat Exchange Institute) method for steam jet vacuum ejectors. The current blog entry presents the power consumption of vacuum pumps based on the method provided in the design guide of a reputed engineering company.

In the present blog entry the air equivalent rate does not use any entrainment ratio correction which is typical for steam ejectors but may not be applicable to vacuum pumps as defined for steam ejectors.

By definition these two should be the same, but I have not compared any results based on these two different methods. However, you are free to check and present your findings.

Regards,

Ankur.

sandiphatkar

Thank you !

rkapahi

Vancouver?

Wow, that's my home town!! I now live in California.

Ankur, can you recommend a spreadsheet for doing basic design calculations for a wet (packed) scrubber? I am working on a project where we have a gas stream laden with volatile organics.  The scrubber will use water as the scrubbing fluid.

Thank you,

Ray

srinu_chems

sir,

how to select the moter gear box for a chemical reactor in chemical industry

regards

ns rao

DrRana

Refer book --> Liquid Ring Vacuum Pumps, Compressors and Systems by H. Bannwarth for more accurate power consumption estimation.

WouterVH

Thanks for elaborating these equations. This information is hard to find in chemical engineering handbooks. Where did you get these equations from?

Do you have a reference to these equations? I don't think you found these in the handbook from H. Bannwarth, or did I overlook them?

Wouter

ankur2061

Hi Wouter,

I haven't referred any handbook. These are from a reputed engineering consulting  company design standards.

Regards,

Ankur

tushthombare

Hi Ankur,

Thank you for nice explanation to find out power consumption of vacuum pump.

I deals with design of steam surface condensers and LRVP system selection for evacuation of surface condenser.

Here I tried to find out power consumption of LRVP system for one of condenser, as below:

T= Temperature of steam being evacuated = 43.8 Degree C

P= Operating pressure of steam being evacuated = 67.8 mm Hga (torr)

MW= Molecular weight of steam being evacuated = 18

G= Flow rate of steam being evacuated = 590970 kg/hr

So, m = G*sqroot((273.5+T)*28.96/(293.15*MW)) = 1027998.178

SF = Size factor = m/P = 1027998.178/67.8 = 15162.215

But as per your above lesson SF range for LRVP is given as 0.02 to 16.

Regards,

Tush

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